This invention relates to cooling of produce to remove field heat from harvested crops such as lettuce, celery, mixed leaf lettuce, spinach, cabbage, and cauliflower. The invention is preferably used in connection with cooling equipment provided by Western Precooling Systems under the trademark Hydro Vac and illustrated in U.S. Pat. Nos. 3,844,132 and 4,576,014. It is envisioned that this equipment will also be used to vacuum cool products other than produce, such as turkeys or sod, that would not use the water spray of the Hydro Vac system.
In this equipment, freshly harvested produce (or other product) is placed in a vacuum chamber equipped with a vacuum pump, vacuum and temperature control instruments and gages, a refrigeration system having a cooling coil inside the chamber for condensing water evaporated from the produce and a spray system for spraying water on the produce. The produce is cooled by evaporation of water from the produce and by conduction of heat from the produce to the condensed water. A restricted vent is provided into the chamber for re-pressurizing the chamber at a controlled rate after cooling so that water on the produce is not forced into the cells of the produce to damage the produce. The heat removed from the produce is removed from the chamber by the refrigeration coils which condense water vapor and cool the air and water in the chamber.
Operation of the vacuum pump in the system can be started and stopped by totally starting or stopping a pump or by opening and closing a butterfly valve between the pump and chamber, and similarly, operation of the refrigeration system can be started and stopped by totally starting and stopping the system or by opening and closing a valve in an ammonia refrigerant line. The cooling system has been manually controlled with the operator:
a--starting the vacuum pump, PA1 b--then starting the Hydro Vac water circulation pump which may be operated or not and for different times depending on the different produce (where water spraying is used, it is usually desirable to start water spraying at a pressure, for instance about 300 millimeters of mercury, above that at which water flashing vaporization starts so that the produce is wet and does not lose its own water when flashing starts), PA1 c--then starting the refrigeration system after the vacuum gauge stops falling indicating that water in the produce has started flashing (this occurs at a pressure of about 15 to 20 millimeters of mercury depending of produce field temperature, 15 mm for 75 degrees F. and 20 mm for 100 degrees F.), PA1 d--then monitoring wet bulb or internal product temperature probes or the vacuum gauge to determine when the surface temperature of the produce reaches about 33 degrees F. and stopping further cooling by first stopping the ammonia supply to the cooling coil in the refrigeration system, then closing the vacuum butterfly valve if necessary, and then flicking the vent valve open and closed quickly if necessary, PA1 e--then restarting the refrigeration and pump if the temperature and pressure in the chamber rise, and PA1 f--finally when the entire load of produce has been cooled to about 33 degrees F. as indicated by the temperature probe in the produce, the process is stopped by stopping the vacuum pump and refrigeration, venting the chamber and removing the produce.
Operation of the Hydro Vac equipment in this way has been very successful, but operator inattention to the equipment has reduced throughput of product and on occasion produced inconsistent results, or even freezing of the product, thereby ruining the product.